1. Field of the Invention
The present invention relates generally to methods and apparatus for connecting tubular bodies. More specifically, the present invention relates to methods and apparatus for effecting a secure mechanical engagement and seal in the connected ends of pipe sections that are expanded radially to form an increased internal diameter pipe string.
2. Description of the Prior Art
A new technique for casing well bores expands the well casing pipe radially after the casing pipe string has been lowered into a well bore. The casing string is enlarged radially by moving an oversized forging tool, or xe2x80x9cpig,xe2x80x9d through the string, causing the string to expand radially beyond its original radial dimensions. This technique allows subsequent strings of casing to be lowered through the previously enlarged casing string sections and thereafter similarly expanded. The result is a well cased by a series of linked sections of casing having substantially the same internal diameters. The procedure is explained in greater detail in U.S. Pat. No. 5,348,095, assigned to Shell Oil Co.
Conventional casing strings are made up of a series of individual pipe joints secured together at their ends by threaded connections. Typically, a joint of casing is approximately 40 feet in length and has a threaded male xe2x80x9cpinxe2x80x9d connection at one end of the joint and a threaded female xe2x80x9cboxxe2x80x9d connection at the opposite joint end. The joint may have a pin at each end, with the box connection being formed by a short coupling threaded onto one of the pin connections. Some casing is made with the box connection integrally formed at one end of the casing joint. These integral box connections may be radially larger than the pipe body, or they may be the same size. In the latter case, the connection is called a flush joint connection.
The threaded engagement between a properly secured pin and box connection in a conventional casing joint is effective in maintaining a secure mechanical connection that holds the string together and effectively seals the internal casing area from the formation well bore environment. When the casing string is enlarged radially, a conventional connection changes dimensionally in a way that can prevent the engaged components of the connection from properly engaging and sealing. The radial expansion of a conventional connection may also weaken or otherwise damage the pin and box structure sufficiently to permit mechanical separation.
Threaded connections for oil field use mainly rely on three types of seals: either metal-to-metal shouldering seals or seals formed by engaged threads with high thread interference using thread compound to effect a seal in the void areas, or deformable seal rings entrapped in the thread area. All three seals of the se types are disabled by the radial expansion caused by the pig. In each case, following the expansion of the pin and box, the pin and box members radially separate, causing the seals to fail.
The end of the expanded pin has a natural tendency to spring back into the internal diameter of the pipe when the pig is removed. The separation and distortion following the expansion process compromises the seals located between the pin and box and creates an obstruction in the bore of the pipe. The distortion can be so great that the pin and box threads may also disengage.
A related cause of the failure of expanded connections is that the radial expansion of a conventional casing joint connection produces different degrees of permanent deformation in the pin and box. When the expanding force is removed, the pin tends to return to its original dimensions more so than does the box. The result is an annular separation between the pin and box that may permit leakage or mechanical separation.
Radial expansion of conventional connectors may also permit the connectors to leak. The sealing mechanism in many threaded pipe connections results from the engagement of metal-to-meal sealing surfaces in the pin and box. The engagement of these seal surfaces closes the annular space between the pin and box to provide a pressure seal. Radial expansion of the connection can distort or displace the sealing surfaces to permit leakage through the annular space.
Connectors that employ an elastomeric, annular seal ring between the engaged surfaces of the pin and box are also subject to leakage when the connection expands radially. The annular elastomeric seal of conventional O-ring-sealed connectors is carried in an annular groove formed in either the pin or the box, or both. The seal of such a connection is formed when the annular seal ring is compressed radially between the fully engaged pin and box. Subsequent radial expansion of the engaged connection changes the radial compression of the annular seal ring, which in turn may permit leakage through the expanded connection. The dimensional changes in the groove occurring during the expansion process may also damage the annular seal ring, permitting the seal to leak.
Conventional well pipe connections are also susceptible to splitting along the length of the box when the connections are expanded radially. The expansion process concentrates stresses of expansion in any thin wall sections present at the ends of the connected pipe segments. The acceptable tolerance for wall thickness in conventional connectors is relatively large so that a pin or box may have a non-uniform thickness that includes relatively thin wall areas without being defective. In this conventional connector, however, the concentration of the stresses induced by expansion of the connections may be sufficient to rupture or over-expand the thin section. The probability of a conventional connector having an area with a relatively thin wall section in either the box or the pin is too great to employ such connectors in pipe strings that are to be radially expanded.
An annular, elastomeric seal is carried in a non-standard annular groove formed within the threaded box of a tubular connector. An annular, spacer ring is positioned adjacent the seal ring within the groove. The dimensions of the groove and the seal ring and spacer ring are functionally related so that the seal ring is compressed axially and radially when the pin of the connection is expanded radially into the box and both the box and pin are then further expanded radially. Radial expansion of the connection causes the groove in the box to contract axially, forcing the spacer ring against the seal ring to compress the seal ring between the spacer and the opposing wall of the groove. The spacer ring assists in retaining the seal ring within the groove and also serves as a compressive wall surface to trap the seal ring and force it to expand within the groove as the groove contracts during the expansion of the connection. The expanded seal ring is extruded into the gap between the pin and box at the groove wall opposite the expansion ring to provide a pressure-tight seal.
A preferred form of the connection of the present invention is equipped with a single-taper thread section having a hooked thread form with no significant radial thread interference between the engaged pin and box components. An internal, reverse angle torque shoulder engages the end of the pin to prevent the pin end from moving radially inwardly away from the expanded box following the expansion of the connection.
The outside diameter of the box adjacent the thin section of the face of the box and the inside diameter of the pin adjacent the thin section end of the pin are machined to limit the variation of the wall thicknesses of the two connection components to prevent concentration of stresses in a thin-walled portion of either of the components during the expansion process.
Where the pig travels from the pin into the box, the pin internal diameter is less than or equal to the box internal diameter. Smooth and gradual transitions between the machined internal diameters and the xe2x80x9cas rolledxe2x80x9d internal diameters on the pin and box connections are also provided to limit the induced stress formed in the connection as it is expanded radially.
An elastomeric, annular seal is installed in a groove in the box between the end of the thread and the reverse angle torque shoulder provided for capturing the pin end. The annular seal is dimensioned to accommodate the radial expansion and axial contraction of the connection during the expansion process while maintaining sealing integrity. An annular spacer ring employed in the groove with the seal ring constrains the movement of the seal ring as the pin enters the box during the connection assembly and as the connection is enlarged during the expansion process to maintain the seal ring in place and to protect it from damage.
If desired, a second elastomeric seal ring may be installed in the pin of the connection between the end of the pin thread and the extreme end of the pin connection to seal against external pressure. The second seal ring is also employed with a spacer ring, and both are carried within a groove formed in the pin, with the groove having dimensions that cooperate with those of the seal ring and spacer ring to cause the seal ring to expand radially as the connection is expanded radially.
The reverse angle torque shoulder provides a positive makeup position reference to assist in assembly of the connection and also provides a mechanism to store torsional energy to resist any unscrewing of the connection after assembly. Additionally, the reverse angle torque shoulder provides radial support to the nose of the pin to prevent the pin from disengaging from the box during the expansion process.
The threads of the connection are dimensioned to provide a means to easily assemble the connection and provide a structural element for supporting the loads encountered in the running and setting of the well string. The thread taper provides a means to maximize the load-bearing sections of the connections.
The provision of free-running threads (no thread interference) enables the connections to be made up with reduced probability of galling of the thread. The use of a hooked thread form provides additional structural support to maintain the mechanical connection between the pin and box during the radial expansion of the connection. The hooked thread form also retains the pin and box together as the pin retracts after expansion. The hooked thread form also provides additional support to assist the pin from radially extending from the box and xe2x80x9cjumping outxe2x80x9d during application of axial tension loads.
An important feature of the present invention is the provision of a controlled thickness over the thin sections of the face of the box employed in the connection. By controlling the variation in the thickness of the thin section at the face of the box, large variations in the thickness are avoided to prevent concentration of strain in the thinner portions, which causes the thin portion to split during the expansion process. Splitting of the thin sections can permit the connection to leak and fail structurally, causing disengagement of the box and pin.
In a preferred form of the invention, at the face of the pin, the internal diameter of the box of the connection is substantially the same as that of the pin. If the box internal diameter is significantly smaller than the pin internal diameter at the internal interface between the pin and box connection, the box will radially expand more than will the pin. This greater radial expansion can significantly reduce, or completely eliminate, the contact of the seal ring between the pin and box, causing leakage. The greater radial expansion of the box may also lessen or eliminate the radial engagement of the thread flanks. A reduction or elimination in the radial engagement of the thread flanks may lead to an inability of the threads to maintain adequate structural support for the applied loads, causing the connection to disengage.
The pin and box connections of the preferred form of the present invention are provided with substantially concentric internal diameters to further ensure that the pin and box are radially expanded to the same extent. The provision of uniform wall thicknesses in the pin and box connectors, as well as the provision of substantially identical internal diameters between the pin and box is ensured by machining the connection in the critical areas to prevent normal variations occurring in conventional pin and box connectors.
Any changes in the internal diameter sections of the connection are held at smooth and gradual transitions to prevent abrupt changes that may disrupt the passage of the expansion device as it moves through the connection.
The annular seal ring of the present invention is positioned between the engaged pin and box in a way to prevent or reduce passage of well bore fluids from the inside of the pipe, even after the connection has been expanded radially. The dimensions of the annular seal ring are selected to maintain an adequate xe2x80x9csqueezexe2x80x9d after the expansion of the connection to create and maintain a desired seal between the engaged pin and box.
The annular backup ring employed in conjunction with the annular seal ring prevents the seal ring from being either partially or completely moved out of the seal assembly groove during the assembly of the connection. The backup ring also prevents the annular seal ring from being partially or completely pushed out of the seal ring groove during expansion of the connection.
If desired, the seal of the present assembly may also be employed as an external seal within a groove in the pin connection between the end of the thread and the external shoulder of the pin. The annular seal ring and spacer ring of the external seal are dimensioned as with the case of the internal seal assembly employed in the box so that the seal ring expands radially into sealing engagement between the pin and box during the expansion process. The primary purpose of the second seal assembly is to provide better resistance to the passage of fluid from the outside of the pipe into the inside of the pipe. Placement of the optional seal ring assembly at this position provides better resistance to the passage of fluid from the outside of the pipe because of its presence in a substantially thicker section of the pin, which provides more resistance to radial separation between the pin and box seal surfaces under pressure loads.
By employing an elastomeric annular seal ring in a groove having dimensions that cause the seal ring to extend radially when the connection carrying the seal is expanded radially, an effective seal is maintained between the expanded pin and box. In addition, by maintaining a uniform thickness around the circumference of the box counterbore by machining the connection or otherwise closely controlling the connection thickness, the effects of plastic strain induced during the expansion process are distributed evenly to prevent splitting of the box connection. The machining step also ensures that a high degree of concentricity is maintained between the internal diameter of the engaged pin and box connections. This permits the forging pig to pass through the connection without hindrance and prevents a high degree of differential expansion between the pin and box connection to prevent disengagement of the threads and/or seals.
Conventional flush joint connections are machined on plain end pipe, with the pipe end being swaged and then machined to a significantly smaller diameter than the pipe. The resulting difference in internal diameter between the pin and box of these connections creates an abrupt change in the connection geometry that can hinder the passage of the pig. The present invention provides a machine surface on the pin""s internal diameter, which is substantially the same diameter as the mating box connection, to prevent the abrupt change in connection geometry.
Casing formed by electric resistance welding, rather than seamless casing, has a more uniform body wall and is preferred for use in an expanded casing string.
From the foregoing, it will be appreciated that an object of the present invention is to provide a method for radially expanding connected tubular bodies to produce a larger internal diameter within the connection, with the enlarged connection being mechanically secure and leakproof.
It is also an object of the present invention to provide a method for radially expanding a threadedly engaged tubular connection to provide a connection that remains securely engaged and leakproof after being expanded.
Yet another object of the present invention is to provide a method for radially expanding a string of joined casing pipe sections within a well bore with each of the expanded joint connections remaining securely engaged and sealed to ensure leakproof support of the string in the well bore.
An important object of the present invention is to provide a seal assembly that can be employed between superimposed, inner, and outer tubular bodies to produce a seal when the inner body is expanded radially into the outer body and both bodies are then together expanded radially.
It is also an object of the present invention to provide an engaged pin and box connection having a seal assembly that can be expanded radially to compress the seal assembly between the pin and box to form a seal in the expanded connection.
Yet another object of the present invention is to provide a seal for a pin and box connection that is compressed axially and radially as the connection is radially expanded to provide a seal between the expanded pin and box.
Another object of the present invention is to provide a pin and box connection in which the pin nose is engaged mechanically and locked with the box to prevent the pin nose from contracting radially away from the box after the connection is expanded radially.
It is yet another object of the present invention to provide a pin and box connection having threads that interlock to prevent mechanical separation of the threads when the connection is expanded radially.
An important object of the present invention is to provide a pin and box connection that is resistant to failure at a thin-walled area of either the pin or the box when the connection is expanded radially.